1. Field of the Invention
The present invention relates generally to an ink-jet printing apparatus. More specifically, the invention relates to an ink-jet printing apparatus for controlling a coloring agent particle in a pigment type ink by electrophoretic effect.
2. Description of the Related Art
In recent years, non-impact printing methods are attracting attention because they generate little noise during printing. An ink-jet printing method is quite dominant its capability of direct printing on a printing medium with a simple mechanism, and of printing on plain paper. Various systems of ink-jet printing apparatus have been proposed. Conventionally, an electrostatic ink jet printing apparatus prints by applying a voltage between an electrode provided on the back surface of the printing medium and a needle shaped ejection electrode, thereby making a coloring agent of an ink or the like fly toward the print medium by an electrostatic force of the generated electric field, as disclosed in Japanese Unexamined Patent Publication Nos. Showa 60-228162 and Heisei 8-309993.
FIG. 7 is a general illustration of the conventional electrostatic ink-jet printing apparatus. The shown conventional ink-jet printing apparatus includes an ink chamber 1 filled with a pigment type ink 10, an electrophoretic electrode 3 for causing coloring agent particles in the pigment type ink to move to concentrate at the ink ejection openings 2 by electrophoretic effect, a plurality of ejection electrodes for ejecting the coloring agent particle concentrated at the ink ejection openings 2, toward a printing medium 4, and an opposing electrode 9 arranged on a back surface of the printing medium 4 in opposition to the ejection electrodes 5.
The ink ejection openings 2 are separated per respective ejection electrodes 5 by flow passage walls 8 so that a convex meniscus of the pigment type ink 10 can be formed at the tip end of respective ejection electrodes. The ink chamber 1 connects to a not shown ink tank through an ink supply port 6 and an ink drain port 7 by a not shown tube. The pigment type ink 10 in the ink chamber 1 is under a back pressure and forced to circulate.
FIG. 8 is a chart of a waveform of a voltage that is applied to the electrophoretic electrode and the ejection electrode of the conventional ink-jet printing apparatus. Operation of the conventional ink-jet printing apparatus will be discussed with reference to FIGS. 7 and 8. The shown ink-jet printing apparatus utilizes electrophoretic effect to orient the coloring agent particles in one direction by applying an electric field to the pigment type ink containing charged coloring agent particles. Namely, by applying a constant voltage VI to the electrophoretic electrode 3 to apply the electric field to the ink chambers 1 filled with the pigment type ink 10, the coloring agent particles in the pigment type ink 10 move toward the ink ejection openings 2 at a certain electrophoretic motion speed to form a convex meniscus of the pigment type ink 1 at the tip ends of the ejection electrodes 5. Ejection occurs by electrostatic force when a pulse of voltage V2 is applied to the ejection electrodes 5 for a pulse period T0 particles move to concentrate to the tip end portion of the
By electrostatic force, the coloring agent particles overcome the meniscus, the surface tension of the pigment type ink, viscosity, and so forth, to fly from the tip end of the ejection openings 5 toward the opposing electrode 9 as fine flying particles at a timing synchronized with the pulse form ejection voltage and to be deposited on a printing medium 4.
A problem encountered in the conventional ink-jet printing apparatus is a possibility of self-ejection causing coloring agent particles to eject without application of the pulse form ejection voltage on the ejection electrodes. Self-ejection degrades the image quality of what is printed.
Before discussion will be given for the cause of the self-ejection, brief discussion will be given for mobility of the coloring agent particle, charge relaxation time, and the time constant of deformation of meniscus.
The mobility .alpha. [(m/s)/V/m)] of the coloring agent particle is generally expressed by .alpha.=.epsilon..zeta./6.pi..mu., wherein .epsilon. is a dielectric constant of a medium, .zeta. is a zeta potential, .mu. is a viscosity. The mobility .alpha. is a characteristic value specific to the pigment type ink used and is used for deriving the speed of motion of the coloring agent particles as they move to concentrate at the ink ejection openings in the electric field generated by a voltage applied to the electrophoretic electrode,
The charge relaxation time is a period required to establish a balanced condition of the influence of the electric field caused in the pigment type ink by the voltage applied to the electrophoretic electrode, for which a ratio of an electric conductivity .sigma. of the pigment type ink and the dielectric constant .epsilon., and a time constant .epsilon./.alpha. may provide references. The time constant .epsilon./.alpha. is also a characteristic value specific to the pigment type ink to be used similar to the mobility .alpha.. For instance, a time constant .epsilon./.alpha. of a pure water is about 1 .mu.s.
The time constant of deformation of meniscus can be an indicia of the condition of variation of shape of the meniscus and is associated with the surface tension and viscosity of the pigment type ink, and the motion speed and degree of concentration of the coloring agent particles. Since the surface tension and the viscosity of the pigment type ink are characteristic values specific to the pigment type in question, they should be constant. Therefore, a primary factor in determining the time constant of deformation of the meniscus is the motion speed and degree of concentration of the coloring agent particles, which is dependent on the a voltage applied to the electrophoretic electrode. Accordingly, by controlling; the voltage applied to the electrophoretic electrode, the time constant of deformation of meniscus can be varied.
The reason will be discussed hereinafter. FIG. 5 shows the influence of the electric field caused in the pigment type ink upon the instantaneous change of the voltage applied to the electrophoretic electrode to a target voltage V1, until establishment of balance. By applying the targeted voltage V1 all at once, the coloring agent particles move toward the ink ejection openings simultaneously at a speed which can be calculated from the foregoing mobility .alpha.. By simultaneous motion of the coloring agent particles toward the ink ejection openings, abrupt concentration of the coloring agent particles causes a meniscus to form. The shape of the meniscus is varied. Due to variation of the shape of the meniscus and certain external factor, the coloring agent particles can be ejected unwantedly. At this time, the time constant of deformation of meniscus to be an indicia of the condition of variation of the shape of the meniscus becomes smaller than a time constant .epsilon./.sigma. as shown in FIG. 6.